Conventional lift cranes include a rotatable body or upper works which supports a lift boom and lift machinery. The upper works rotates about a vertical axis on a lower works or base. If the crane is mobile, the lower works is typically crawler mounted. The lifting capacity of a mobile crane is largely determined by the geometry of the base, since all the compression and tilting loads must act through and around the mobile base to the ground. Larger cranes have been provided with increasing amounts of counterweight carried on the rotatable upper works to resist the overturning moment of the larger loads.
Earlier efforts to provide a lift enhancing beam attachment for lift cranes include U.S. Hamson Patent U.S. Pat. No. 3,435,961 issued Apr. 1, 1969, U.S. Juergens Patent U.S. Pat. No. 4,258,852 issued Mar. 31, 1981, and U.S. Petzold et al. Patent U.S. Pat. No. 4,729,486 issued Mar. 8, 1988. In the Juergens Patent, an auxiliary counterweight arrangement is provided consisting of a V-shaped attachment including two weight carriers at the end of carrier arms. The carrier arms are comprised of connected modular units. The number of units can be increased or decreased to lengthen or shorten the carrier arms, thereby increasing or decreasing the moment arm of the counterweight carriers attached to the end of the arms. Auxiliary counterweights are provided on the counterweight carriers at the ends of the arms, and are used in addition to the counterweights mounted on the crane upper works.
U.S. Petzold et al. Patent discloses an attachment for increasing the lifting capacity of a crane. The attachment comprises a support beam of fixed length. A counterweight assembly including a moveable carrier is mounted on the support beam, and means is provided for moving the carrier along the beam. A reference is also made, at column 4, lines 29 through 35, to sensor means "provided for indicating when a support leg is lifted from the ground and a suitable signal transmitted to the operator's cab", and a load indicator "incorporated in the crane and utilized for positioning the counterweight on the beam in accordance with the load being lifted".
Both of these patents are generally directed to attachments for larger cranes, and are not intended to provide a versatile crane attachment for use with smaller sized cranes.
U.S. Hamson Patent discloses a crane including a counterweight slidably mounted on a crane upper works, extendable outwardly opposite a boom, and doubled-ended means for extending and retracting the counterweight.
Attention is also directed to Russian Patent No. 214,777.